Additive for the prevention of nitrosamine formation in silicone articles

ABSTRACT

The present invention relates to a method for making nitrosamine-free silicone articles by treating the mixture of vinyl containing organopolysiloxane and filler with hindered phenols, a nitrosamine-free composition and a nitrosamine-free article made therefrom. A preferred nitrosamine-free molded article is a baby bottle nipple.

This is a divisional of application Ser. No. 08/670,474 filed on Jun.26, 196, U.S. Pat. No. 5,959,025 which is a divisional of Ser. No.08/350,935, now issued U.S. Pat. No. 5,543,457, filed on Dec. 7, 1994.

The present invention relates to production of nitrosamine-free siliconearticles. More particularly, it is concerned with production ofnitrosamine-free silicone articles by the addition of additives.

BACKGROUND OF THE INVENTION

Several reports have been published describing the presence of volatileN-nitrosamines in various rubber products. The present concern about theoccurrence of volatile N-nitrosamines in baby bottle rubber nipples andthe possible migration of these compounds into infant formula wasprompted by a report of Preussmann et al., (1981) Am. Chem. Soc. Symp.Ser. 174, American Chemical Society, Washington, D.C., p. 217.

A method was described for the estimation of volatile N-nitrosamines inthe rubber nipples of baby bottles. In a study of rubber nipples fromone manufacturer, N-nitrosodimethylamine, N-nitrosodiethylamine andN-nitrosopiperidine were determined by gas chromatography, using athermal energy analyzer, and their presence was confirmed by massspectrometry with average levels of individual nitrosamines ranging from22 to 281 ppb. When the nipples were sterilized in a conventionalsterilizer together with milk or infant formula the three nitrosaminesmigrated into the milk or formula. Storing a bottle of milk with arubber nipple inverted in it for 2 hr at room temperature or overnightin a refrigerator after sterilization resulted in an 8-13% averageincrease in the nitrosamine levels migrating into the milk. On repeatedsterilization of a single nipple, the quantities of nitrosaminesmigrating into milk from rubber nipples declined steadily, but afterseven sterilizations, nitrosamines were still readily detectable in themilk. Nitrosamine levels were higher in rubber nipples aftersterilization, indicating the presence of nitrosamine precursors in thenipples. No nitrosamines were found in raw, uncured rubber. Chemicalaccelerators and stabilizers added during the organic rubbervulcanization process are the source of the amine precursors in rubbernipples.

On Jan. 1, 1984, the U.S. Food and Drug Administration (hereinafter"FDA") established an action level of 60 ppb total N-nitrosamines inrubber nipples. The action level was reduced to 10 ppb on Jan. 1, 1985.

A collaborative study was conducted on the FDA dichloromethaneextraction method for determining volatile N-nitrosamines in baby bottlesynthetic and natural organic rubber nipples. Following dichloromethaneextraction, N-nitrosamines were determined by gas chromatography-thermalenergy analysis. Six pairs of blind duplicate rubber nipple samplesrepresenting 6 lots were analyzed by 11 collaborating laboratories. Allsamples were portions taken from equilibrated composites of cut-uprubber nipples obtained from manufacturers in the United States.Recoveries of the internal standard (N-nitrosodipropylamine) atapproximately 20 ppb ranged from 10 to. 120%. Reproducibility relativestandard deviations (RSD) were between 35 and 45% for N-nitrosaminelevels from 10 to 20 ppb. However, when data from laboratories withrecoveries less than 75% were excluded (this is now specified in themethod), RSD, values were between 11 and 32% for N-nitrosamine levelsfrom 6 to 26 ppb. Values were consistent with or better than thosereported for other analytical techniques designed to quantitate tracecontaminants at the low ppb level, e.g., aflatoxin in food. The methodhas been adopted as an official first action for the quantitation ofvolatile N-nitrosamines in baby bottle rubber nipples. See GasChromatographic-Thermal Energy Analysis Method for Determination ofVolatile N-Nitrosamines in Baby Bottle Rubber Nipples: CollaborativeStudy, by Gray & Stachiw, J. Assoc. Off. Anal. Chem. (1987) 70, MarchIssue.

Various additives have been reported to prevent nitrosamine formation.In one case the order of addition of reagents was found to be key toprevention of nitrosamine formation in rubber and the additiveerythorbic acid was employed. T. Fukuda, Y. Matsunra, S. Kusumoto,Nippon Shokuhin Kogyo Gakkaishi, 28 (1981) 606, CA 96, 85000e. Sorbicacid and sodium ascorbate were reported to be effective at preventingnitrosamine formation. See K. Tanaka, K. C. Chung, H. Hayatsu, T. Kada,Food Cosmet. Toxicol., 16 (1978) 209 and R. L. S. Patterson, D. S.Mottram, Abstr. Commun. Eur. Meet. Meat Res. Work., 20th An ForasTuntais: Dublin, Ire. (1974) 77, CA 85, 19272s. Alcohols in generalinhibit nitrosamine formation at pH<7 while hindered alcohols such asbutylated hydroxy anisole (BHA) was reported to inhibit the formation ofnitrosamine under conditions of oxidation of amines by peroxide. See T.Kurechi, K. Kikugawa, T. Kato, Food Cosmet. Toxicol., 18 (1980) 591, CA94, 191596x and N. S. Shehad, PCT Int. Appl. WO 9322273, CA 120,191111r.

Although research in the synthetic and natural organic rubber industryhas been devoted to lowering or eliminating nitrosamines, none of thesestudies have included silicone rubber materials. Silicone elastomericcompositions, in contrast to synthetic and natural organic rubbercompounds, can be prepared from a vinyl-containing polydiorganosiloxane,an organohydrogensilicone crosslinker, and a platinum catalyst. Thecompositions of this type are desirable for many reasons. For instance,they cure without by-products. They can cure at room temperature or atelevated temperatures. They can be stabilized for storage at roomtemperature by utilization of a platinum catalyst inhibitor. They can bemade from high and low viscosity polymers. These compositions utilizecomponents that are low enough in viscosity that they are easilypumpable or extrudable as well as have a fast cure time. Thesecompositions also provide cured silicone elastomers which arecharacterized by high strength and high durometer. Thus, these siliconesbecame leading candidates to replace the organic rubber compounds.

Analysis of the cured silicone elastomers showed no presence ofnitrosamines. However, upon post-baking as required by FDA and EuropeanBGA, the presence of nitrosamines was detected. When the cured sheet ispost-baked, open (PBO) at 200° C., 3-7 ppb dimethylnitrosamine (DMNA) isdetected. If the sheet is first wrapped in aluminum foil and then postbaked, sealed, (PBS) the sheets had from 30-70 ppb DMNA.

For silicones to serve these FDA regulated markets, Applicants developedvarious techniques to prevent formation of nitrosamine during post bake.In co-pending application Ser. No. 08/298,981 filed Aug. 31, 1994,applicants indicated that post baking in an atmosphere substantiallyfree of oxygen can prevent formation of nitrosamine during post baking.In another co-pending application Ser. No. 08/298,728 filed Aug. 31,1994, applicants reported that treating the polymer/filler mixture withan effective amount of acid can also prevent the formation ofnitrosamine. Although these methods have been proven to be effective,not all post baking facilities are equipped with inert atmospherecapacities. The use of acid can be corrosive to some equipment.Therefore, applicants continued to look for a new method.

SUMMARY OF THE INVENTION

There is provided by the present invention a nitrosamine-free cured andpost-baked silicone composition comprising an effective nitrosamineformation inhibiting amount of a (E) hindered phenol admixed with acurable silicone elastomer, cured and post-baked. There is also providedan article made from such nitrosamine-free silicone composition as wellas a method for making such a silicone composition.

A preferred nitrosamine-free composition when cured and post-bakedcomprises an effective nitrosamine formation inhibiting amount of a (E)hindered phenol admixed with a (A) vinyl-containing organopolysiloxane,a (B) silicon hydride siloxane, a (C) filler and an (D) effectivecatalytic amount of an addition-cure catalyst.

The method of the present invention comprises admixing component (E)with a curable silicone composition comprising Components (A), (B), (C)and (D), curing the composition comprising Components (A), (B), (C), (D)and (E), and post-baking the composition comprising Components (A), (B),(C), (D) and (E) whereby a nitrosamine-free article is obtained. In themethod of this invention, the order of addition of a (E) hindered phenolto the other Components (A), (B), (C) or (D) is not critical andComponent (E) may be added to any of Components (A), (B), (C) or (D) orany combination or mixture thereof. The various combinations ofComponents (A), (B), (C) or (D) are well known to the artisan skilled inthe art of making curable silicone compositions and are not a part ofthe present invention. The nitrosamine-free article of the presentinvention is the product of the method of the present invention andcomprises a composition of the present invention.

The critical feature that led to this invention is based on thediscovery that the addition of hindered phenols such as butylatedhydroxy toluene (BHT) prevents formation of nitrosamine upon post bakingof a cured silicone article.

DETAILED DESCRIPTION OF THE INVENTION

Component (A), the vinyl-containing organopolysiloxanes, generally has aviscosity of from 5,000 to 1,000,000 centipoise at 25° C. Preferredvinyl-containing organopolysiloxanes may be vinyl-stopped polymerexemplified by the general formula M^(Vi) D_(x) M^(Vi), a vinyl-on-chaincopolymer exemplified by the formula MD^(Vi) _(x) D_(y) M, avinyl-stopped, vinyl-on-chain copolymer exemplified by the formulaM^(Vi) D_(x) D^(Vi) _(y) M^(Vi), vinyl and trimethylsilyl-stoppedcopolymers exemplified by the formula MD_(x) M^(Vi), or mixturesthereof, wherein Vi represents a vinyl radical, M represents atrimethylsiloxy radical, M^(Vi) represents dimethylvinylsiloxy, Drepresents dimethylsiloxy, D^(Vi) represents methylvinylsiloxy and "x"and "y" are positive integers. Such polymers are taught by U.S. Pat. Nos5,082,886, 4,340709, 3,884,866 issued to Jeram et al., U.S. Pat. No.5,331,075 issued to Sumpter et al., U.S. Pat. No. 4,162,243 issued toLee et al., U.S. Pat. No. 4,382,057 issued to Tolentino, and U.S. Pat.No. 4,427,801 issued to Sweet, hereby incorporated by reference.

Component (B), the silicon hydride siloxane polymer or silicon hydridesiloxane fluid used in the invention can have about 0.04 to about 1.4%by weight of chemically combined hydrogen attached to silicon.

One form of the silicon hydride siloxane is a "coupler" having theformula, ##STR1## where R¹ is selected from C₁₋₁₃ monovalent substitutedand substituted hydrocarbon radicals free of olefinic unsaturation and nis an integer having a value sufficient to provide the "coupler" with aviscosity of 1 to 500 centipoises at 25° C. and from about 3 to 9 molepercent of chain-stopping diorganohydride siloxy units, based on thetotal moles of chemically combined siloxy units in the silicon hydridesiloxane fluid.

In addition to the above silicon hydride coupler formula, the siliconhydride siloxane fluid used in the heat curable organopolysiloxanecompositions of the present invention also can include silicon hydrideresins consisting essentially of the following units, ##STR2##chemically combined with SiO₂ units, where the R² +H to Si ratio canvary from 1.0 to 2.7. Silicon hydride resin also can have units of theformula, ##STR3## chemically combined with SiO₂ units and (R⁴)₂ SiOunits, where the R³ +R⁴ +H to Si ratio can vary from 1.2 to 2.7, whereR², R³ and R⁴ are each independently C₁₋₁₃ monovalent substituted andunsubstituted hydrocarbon radicals free of olefinic unsaturation.

The silicon hydride siloxane fluid also can include linear hydrogencontaining polysiloxane having the formula, ##STR4## where R⁵ is a C₁₋₁₃monovalent substituted and unsubstituted hydrocarbon radical free ofolefinic unsaturation, selected from R¹ radicals, and p and q areintegers having values sufficient to provide a polymer having aviscosity of from 1 to 1,000 centipoises at 25° C.

In the formulas and the chemically combined units described above, R¹,R², R³, R⁴ and R⁵ preferably can be the same or different radicalsselected from the group consisting of alkyl radicals of 1 to 8 carbonatoms, such as methyl, ethyl, propyl, etc.; cycloalkyl radicals such ascyclohexyl, cycloheptyl, etc.; aryl radicals such as phenyl, tolyl,xylyl, etc.; and haloalkyl radicals such as 3,3,3-trifluoropropyl.

Component (C), the filler is any reinforcing or extending filler knownin the prior art. In order to get the high tensile strength, forexample, a reinforcing filler is incorporated. Illustrative of the manyreinforcing fillers which can be employed are titanium dioxide,lithopone, zinc oxide, zirconium silicate, silica aerogel, iron oxide,diatomaceous earth, calcium carbonate, fumed silica, silazane treatedsilica, precipitated silica, glass fibers, magnesium oxide, chromicoxide, zirconium oxide, aluminum oxide, alpha quartz, calcined clay,asbestos, carbon, graphite, cork, cotton, synthetic fibers, etc.

Preferably, the filler is either a fumed or precipitated silica that hasbeen treated. The treating process may be done in accordance with theteachings of U.S. Pat. No. 4,529,774 issued to Evans et al., U.S. Pat.No. 3,635,743 issued to Smith, U.S. Pat. No. 3,847,848 issued to Beers;hereby incorporated by reference, Alternatively, and most preferably,the filler is treated in-situ; that is the untreated silica filler andthe treating agents are added to the silicone elastomer compositionseparately, and the treatment process is accomplished simultaneouslywith the mixture of the filler into the elastomer. This in-situ processis taught by Evans in U.S. Pat. No. 4,529,774; hereby incorporated byreference.

Alternatively, the fillers can be replaced by the vinyl treated silicafiller of U.S. Pat. No. 4,162,243 issued to Lee et al.; and U.S. Pat.No. 4,427,801 issued to Sweet; hereby incorporated by reference.

Component (D), the addition-cure catalyst, is any compound that promotesthe hydrosilation reaction between a silicon hydride and anethylenically unsaturated polyorganosiloxane. Typically, it is aprecious metal compound; usually platinum or rhodium, preferablyplatinum.

Such catalysts are well known in the art Preferred catalysts are taughtby in U.S. Pat. Nos. 3,917,432, 3,197,433 and 3,220,972 issued toLamoreaux, U.S. Pat. Nos. 3,715,334 and 3,814,730 issued to Karstedt,and U.S. Pat. No. 4,288,345 issued to Ashby et al., hereby incorporatedby reference.

Since mixtures containing Components A, B, and C with a Component Dcatalyst, when the catalyst selected is some well known platinum orrhodium containing catalyst, may begin to cure immediately on mixing atroom temperature, it may be desirable to inhibit the action of theplatinum or rhodium containing catalyst at room temperature with asuitable inhibitor if the composition is to be stored before molding.Such catalyst inhibitors are used to retard the catalytic activity ofthe catalyst at room temperature, but allow the catalyst to catalyze thereaction between Components A, B and C at elevated temperature.

One suitable type of catalyst inhibitor is described in U.S. Pat. No.3,445,420 issued to Kookootsedes et al. which is hereby incorporated byreference to show certain acetylenic inhibitors and their use. A 20preferred class of acetylenic inhibitors are the acetylenic alcohols,especially 2-methyl-3butyn-2-ol.

A second type of catalyst inhibitor is described in U.S. Pat. No.3,989,667 issued to Lee et al. which is hereby incorporated by referenceto show certain olefinic siloxanes made by the reaction of secondary ortertiary acetylenic alcohols with siloxanes havingsilicon-bonded-hydrogen atoms, their preparation and their use asplatinum catalyst inhibitors.

A third type of catalyst inhibitor is a polymethylvinylcyclosiloxanehaving three to six methylvinylsiloxane units per molecule.

The optimum concentration of catalyst inhibitor is that which willprovide the desired storage stability at ambient temperature withoutexcessively prolonging the time interval required to cure thecompositions at elevated temperatures. This amount will vary widely andwill depend upon the particular inhibitor that is used, the nature andconcentration of the platinum-containing or rhodium-containing catalystand the nature of the organohydrogensiloxane.

Preferred hindered phenol antioxidants employed as Component (E)include, but are not limited to 2, 6-disubstituted phenols, bisphenols,polyphenols, substituted hydroquinones and substituted hinderedanisoles.

Exemplary preferred 2,6-disubstituted phenols have the formula: ##STR5##wherein R₆ is typically a sterically bulky group such as i-propyl,t-butyl, amyl and the like. R₇ and R₈ can be H, C1-C18 alkyl, aryl, halosuch as chloro, bromo iodo, and nitro. Examples of the 2,6-disubstitutedphenols include, but are not limited to 2,6-di-t-butyl-4-methylphenol(BHT), 2-t-butyl-4-methoxy phenol, 3-t-butyl-4-methoxy phenol,4-(hydroxymethyl)-2,6-di-t-butyl phenol, and styrenated phenols.

Exemplary preferred bisphenols have the formula: ##STR6## wherein R₆ andR₇ are defined the same as above. Examples of the bisphenols include butare not limited to 2,2'-methylene bis (4-methyl-6-t-butyl phenol),2.2'-methylene bis(4-ethyl-t-butyl phenol), 4,4'-methylene bis(2,6-di-t-butyl phenol) and 2,2'-methylenebis(4-methyl-6-(1-methylcyclohexyl)phenol).

Examplary preferred polyphenols include, but are not limited to tetrakis(methylene 3-(3,5-di-t-butyl-4-hydroxyphenyl) propionate) methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl) benzene, and1,3,5-tris (3,5-di-t-butyl-4-hydroxybenzyl) isocyanurate.

Exemplary preferred substituted hydroquinones have the formula ##STR7##wherein R₆ and R₇ are defined the same as above. Examples of thesubstituted hydroquinones include, but are not limited to 2,5-di-t-amylhydroquinone and t-butyl hydroquinone.

An example of the substituted hindered anisoles includes, but is notlimited to butylated hydroxy anisole (BHA).

Compositions of the present invention can be used in a liquid injectionmolding process in which the composition is injected into light weightmolds under low pressures, such as 600 kPa cylinder pressure. Suchcompositions can be cured very rapidly in a hot mold and removed withoutcooling the mold. The type of molding, extruding or curing process usedis not critical and can include any of the processes known in the art Anadvantage of the compositions of this inventions is their extrudabilitywhich makes these compositions adaptable to molding processes such asliquid injection molding at low pressures. The prepared compositionshave a viscosity such that at least 45 grams per minute can be extrudedthrough a 3.175 millimeter orifice under a pressure of 620 kilopascals.Preferably, the viscosity is such that at least 50 grams per minute canbe extruded.

The silicone elastomeric compositions can readily be prepared inconventional mixing equipment because of their fluid nature. The orderof mixing is not critical particularly if the addition-curablecomposition is to be used immediately. However, as is well-known to theskilled artisan, it is preferable to combine (A), (C) following theadditive treatment and thereafter add (D) and (B). This permits thesmall amount of (D) to become well dispersed in (A) and (C) prior to thebeginning of any curing reaction. A suitable two packageaddition-curable composition can be made using such a well-knowntechnique. For example, a convenient two package composition can beprepared by mixing part of (A), part of (C) and all of (D) in onepackage and the remainder of (A) and (C) and all of (B) in a secondpackage such that equal amounts of package one and package two can bemixed to produce the compositions of this invention. Single packagecompositions can be prepared by mixing (A),(B), (C), (D), and a platinumcatalyst inhibitor. These inhibited compositions can be stored forextended periods of time without curing, but the compositions will stillcure when heated above 70° C., preferably when heated above 100° C. toshorten the cure time. As noted above, the hindered phenol can be addedat any stage and to any component or any mixture of Components (A)-(D).

In order to demonstrate various features of this invention, thefollowing examples are submitted. They are for illustrative purposes andare not intended to limit in any way the scope of this invention.

EXAMPLE 1

Test Specimen Preparation

A silicone LIM base compound was prepared according to the teachings ofthis invention using the formulation of Table I.

                  TABLE I                                                         ______________________________________                                        Parts         Inputs                                                          ______________________________________                                        64.5   pts    40,000 cps vinyl chainstopped polydimethylsiloxane                  polymer                                                                     25 pts 325 m.sup.2 /gm octamethylcyclotetrasiloxane treated                     fumed silica                                                                1 pt vinyltriethoxysiloxane                                                   6 pts hexamethyldisilazane                                                    6 pts water                                                                   4 pts 500 cps vinyl chainstopped, polydimethyl,                                 methylvinyl copolymer                                                       4 pts 500 cps trimethylsilyl and dimethylvinyl chainstopped                     polydimethylsiloxane polymer                                                2.5 pts MQ resin                                                            ______________________________________                                    

The 40,000 cps vinyl chainstopped polymer, water andhexamethyldisilazane were mixed together in a cooled mixer. The 325 m²/gm D₄ treated filler was added slowly into the mixture and mixed untilit was completely incorporated. The vinyltriethoxysilane was added intothe mixture and mixed well. The mixer was sealed and heated for one hourat 70-80° C. The batch was stripped at 140° C. under full vacuum toremove all the filler treating reaction by products. BHT was then added(see Table 2) and then the mixture was cooled to 80° C. and added thetwo 500 cps vinyl containing copolymers and mixed well. 2.5 parts of theMQ resin release agent was added. Vacuum was then applied to deair thebatch.

Component A was prepared by adding sufficient amount of Karstedtplatinum organosiloxane complex to obtain 2040 ppm Pt as platinum.Component B was prepared by adding approximately 330 ppm H of hydridecrosslinker (M^(H) D_(x) D_(y) ^(H) M^(H)) and approximately 0.4 partsmethyl butynol, mixed until well dispersed. A LIM composition wasprepared by mixing 100 parts of Component A with 100 parts of ComponentB in a static mixer with no air being introduced. The A/B mixture wasthen molded 20 seconds at 375° F. into 3"×5"×0.070" sheets. The asmolded sheet has less than 1.0 ppb DMNA.

EXAMPLE 2

A sheet prepared according to Example 1 was post baked for one hour@200° C. in an air circulating oven and cooled to room temperature. Thesample was referred to as PBO. A second sheet was wrapped in aluminumfoil and post baked under the same conditions. The sample was referredto as PBS. The results are shown below:

                  TABLE 2                                                         ______________________________________                                                       Nitrosamine (ppb)                                              Amount of BHT (g)*                                                                           PBO         PBS                                                ______________________________________                                        0 (control)    4.0         42.8                                                 1 <1 <1                                                                       0.5 <1, Recheck 2.0 <1, Recheck 1.7                                           0.25 8.8 1.0                                                                  0.1 12.1 1.4                                                                ______________________________________                                    

^(*) Based on a 200 g batch size. All samples were post baked 1h,200°C., in air, PBS samples were wrapped in aluminum foil.

The results clearly demonstrate that the treatment with 0.1-0.25 g BHTsubstantially reduced the formation of nitrosamine. 0.5-1 g BHT totallyeliminated the formation of nitrosamine.

What is claimed is:
 1. A method for making a nitrosamine-free siliconecomposition comprising:(A) mixing an effective nitrosamine formationinhibiting amount of a hindered phenol with a curable siliconecomposition, (B) curing the silicone composition containing the hinderedphenol, and (C) post-baking the cured composition.
 2. The method ofclaim 1, wherein the hindered phenol is selected from the groupconsisting of a 2,6-disubstituted phenol, bisphenol, polyphenol, asubstituted hydroquinone and a substituted hindered anisole.
 3. Themethod of claim 2, wherein the 2,6-disubstituted phenol is2,6-di-t-butyl-4-methylphenol, 2-t-butyl(4-methoxy phenol,3-t-butyl-4-methoxy phenol, 4(hydroxymethyl)-2,6-di-t-butyl phenol orstyrenated phenol.
 4. The method of claim 2, wherein the bisphenol is2,2'-methylene bis(4-methyl-6-t-butyl phenol), 2,2'-methylenebis(4-ethyl-6-t-butyl phenol), 4,4'-methylene bis (2,6-di-t-butylphenol) or 2,2'-methylene bis (4-methyl-6-(1-methylcyclohexyl)phenol).5. The method of claim 1 wherein the composition is cured in a mold andthe mold forms the composition into a baby bottle nipple.
 6. A methodfor making a nitrosamine-free molded article which comprises:(A) mixingan effective nitrosamine formation inhibiting amount of a hinderedphenol and a curable silicone composition, (B) curing the siliconecomposition containing the hindered phenol in a mold, and (C)post-baking the cured molded article.
 7. The method of claim 1, whereinthe curable silicone elastomer comprises(A) a vinyl-containingorganopolysiloxane, (B) a silicone hydride siloxane, (C) a filler, and(D) an effective catalytic amount of an addition-cure catalyst.
 8. Themethod of claim 7, wherein the filler is a reinforcing filler.
 9. Themethod of claim 8, wherein the filler is fumed silica.
 10. The method ofclaim 7, wherein the catalyst is a platinum or rhodium compound.
 11. Themethod of claim 10, wherein the catalyst is a platinum compound.
 12. Themethod of claim 1, wherein the polyphenol is tetrakis(methylene3-(3,5-di-t-butyl-4-hydroxyphenoyl)propionate)methane,1,3,5-trimethyl-2,4,6-tri(3,5-di-t-butyl-4-hydroxybenzyl)benzene, or1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate.
 13. The methodof claim 1, wherein the hindered anisole is butylated hydroxy anisole.14. The method of claim 1, wherein the nitrosamine-free siliconecomposition is substantially transparent.